1. Field of the Invention:
The present invention relates to a surface-acoustic-wave convolver, and more particularly to a matching circuit to be associated with a surface-acoustic-wave device (hereinafter abbreviated as SAW device.)
2. Description of the Prior Art:
A matching circuit having an inductance L, a capacitance C and a resistance R is used for the purpose of matching on SAW device to an associated exterior circuit. For example, Japanese Patent Application Public Disclosure No. 58-158015 shows an SAW device having an inductance connected thereacross. This arrangement can cancel the imaginary part of the admittance of the SAW device. In order to effect the necessary matching of the real part of the admittance of the SAW device, for instance, the conductance of 50 ohms, however, it is necessary that the radiation conductance G of the electrode is designed to be equal to 20 mS, and accordingly the electrode opening must be enlarged. As a result the piezoelectric substrate is enlarged in area.
Japanese Patent Application Public Disclosure No. 61-230512 shows that a series-and-parallel arrangement of capacitive and inductive elements constitutes a matching circuit, which can cancel the imaginary part of the admittance of the SAW device and at the same time, can effect the matching of the real part of the admittance of the SAW device. The printing of the series-and-parallel arrangement of capacitive and inductive elements on a piezoelectric substrate will disadvantageously cause enlargement of the substrate.
There are two different feeding systems to the opposite interdigital transducers (hereinafter abbreviated as IDT) of an SAW device, balance feeding system and unbalance feeding system. These different systems have merits and demerits. Specifically, the balance feeding system in which the opposite IDTs are electromagnetically coupled in balance, has a cancelling effect thus advantageously preventing spurious signals. However, it is necessary to provide a balanced-to-unbalanced transformer between each IDT of the SAW device and an exterior signal channel, which is generally of unbalance type. In contrast to this the unbalance feeding system requires no balanced-to-unbalanced transformer, but there must be provided extra means to suppress spurious signals.
The admittance of the IDT formed on the piezoelectric substrate is given at its center frequency as follows: EQU Y=G+j .omega.C.sub.T
C.sub.r depends on how the IDT is formed on the piezoelectric substrate. Specifically, assume that the IDT is formed on a thin piezoelectric substrate having a conductive lining on its bottom surface, and then in a balance feeding system if used, the value of C.sub.T is reduced two times as much as that in forming the IDT on a thick piezoelectric substrate having no conductive lining at its bottom. Such reduction is attributable to the grounding function of the lining conductor for high-frequency signals. Also, the former arrangement causes an increase of the electromechanical coupling coefficient K.sup.Z. Still advantageously, the balance feeding which the former arrangement adopts, has an effect to suppress spurious signals.
Usually in an SAW convolver having a piezoelectric substrate such as Z.sub.n O substrate, a balance feeding system is used for the above described reason, and also a matching circuit and a balanced-to-unbalanced transformer are used. The use of the matching circuit and the balanced-to-unbalanced transformer, however, prevents disadvantageously the reduction of the size and cost of the device.